The present invention relates generally to the field of photovoltaic device manufacture and in particular, the invention provides a self aligning method of metallization formation in a photovoltaic device.
Many approaches exist for diffusing dopants of one polarity into the surface of a substrate doped with the opposite polarity so as to form a p-n junction. One such approach involves the deposition of a dielectric containing the appropriate dopants onto the surface of the oppositely doped substrate followed by appropriate thermal treatment to allow the dopants from within the dielectric to diffuse into the substrate surface. Examples of techniques for carrying out the thermal treatment include conventional quartz tube furnaces infrared belt furnaces rapid thermal anneals, and the use of lasers.
It is also well documented in the literature, that spatially selective emitters can be important for the achievement of high efficiencies. Heavy doping beneath the metal contact is important, both for reducing contact resistance and also to minimise the contribution made to the dark saturation current for the solar cell by the silicon/metal interface region. For the latter, the heavy doping makes it possible to locate the high surface recombination velocity region associated with the metal/silicon interface more than a minority carrier diffusion length away from the p-n junction, thereby minimising the contribution made to the device dark saturation current by the metal contact. The highest performance commercially available solar cells achieve the heavy doping under the metal by forming a groove through a lightly diffused emitter at the top surface and then subsequently diffusing large amounts of dopant into the walls of the exposed grooves prior to plating the metal contacts also within the grooves where the heavy doping exists. This provides the advantages described above while leaving the majority of the top surface lightly diffused, therefore preventing the formation of a surface dead layer which would otherwise exist if the heavy doping extended across the light receiving surface. The lightly diffused emitter in conjunction with adequate surface passivation enables the achievement of high carrier collection probabilities for carriers generated throughout the depth of the lightly diffused emitter. The disadvantages of this approach include the necessity for two independent diffusion processes, which adds cost and complexity and also possible degradation of the substrate quality through the long high temperature exposure needed to achieve the heavy doping within the grooved region. A third disadvantage of this approach results from the necessity for a damage/debris removal etch following the grooving process to prepare the silicon surfaces within the groove for the subsequent diffusion.
Another approach for achieving the selective emitter with heavy doping beneath the metal contact has been reported by U.Besi-Vetrella et alia (xe2x80x9cLarge area. Screen Printed Silicon Solar Cells with selective Emitter made by Laser Overdoping and RTA Spin-on Glassesxe2x80x9d. 26th IEEE Photovoltaic Specialists Conference, Anaheim. 1997, p.135.). This approach involved the use of a spin-on diffusion source to facilitate formation of the lightly diffused emitter followed by the use of a laser to heat localised regions of the silicon surface at which heavy doping is desired. The pattern formed by the laser is identical to that required for the metallization but with a major complication being the subsequent difficulty of aligning the metal to these heavily diffused regions. Besi-Vetrella et alia, have developed sophisticated techniques for carrying out this alignment for the purposes of screen printing the metal paste onto the heavily diffused regions. A further complication with this work includes the fact that regions of the screen printed metal extend beyond the heavily diffused regions onto the lightly diffused regions with detrimental consequences in increasing the device dark saturation current and potentially leading to the problem of metal penetrating to the p-n junction during heating and perhaps even causing the shunting of the entire device. The latter problem could be overcome by ensuring the heavily doped lines formed by the laser are wider than the screen-printed metal lines. They would need to be wider by a sufficient margin to allow for any alignment tolerances associated with aligning the screen printed metal to the heavily diffused regions over the entire length and width of the solar cell surface. Again however there are detrimental consequences to this approach, including increased dark saturation current resulting from the substantially larger volumes of heavily doped material, the formation of surface dead layers where the heavy doping extends beyond the edge of the screen printed metal, and the increased laser processing necessary to achieve the much greater line widths. In combination, these disadvantages would lead to lower performance and higher cost, compared to a self aligned approach for the metal contact formation if the latter were achievable. The final disadvantage relates to surface passivation. The lightly diffused surface regions require adequate surface passivation to achieve good performance while the described dielectrics which can potentially passivate the surface, can also subsequently cause problems when trying to make contact between the screen printed metal and the heavily doped silicon. A possible solution is to remove the dielectric prior to screen-printing, carry out the screen-printing process, and then subsequently re-passivate the surface without causing damage to the metallization. Even if this is achievable, it adds considerable cost and complexity to the process.
The present invention provides a self aligning method of forming contact metallization in a solar cell, the method including the steps of:
a) on a surface of a semiconductor substrate of a first dopant type, forming a continuous layer of dopant source material of a second dopant type of opposite dopant polarity to that of the first dopant type, the source material being selected to also act or to be made able to act as a surface passivation layer and a metallization pattern mask;
b) thermally treating the dopant source and the semiconductor surface carrying the dopant source, whereby a surface region of the second dopant type is formed in the semiconductor material, the first and second doped semiconductor types forming a p-n junction beneath the surface of the semiconductor substrate;
c) locally heating the dopant source and the underlying semiconductor surface to cause melting of the surface region in zones where metallization is required to contact to the surface region whereby the melted zones of the semiconductor surface region are more heavily doped from the dopant source and the overlying dopant source material is disrupted to expose the more heavily doped surface zones;
d) forming metallization over the heavily doped surface zones such that connection to the surface region of the semiconductor material is made through the disruption in the dopant source layer to the heavily doped surface zones.
Preferably, the source material will also be chosen to form an antireflection coating.
In one embodiment, the dopant source layer is a single layer of dielectric material carrying a dopant source whereby the dielectric material acts as a dopant source, a passivation layer, a metallization mask and perhaps also an anti reflection coating, however, in other embodiments, a doped passivation layer is provided with a dielectric layer formed over it to act as a metallization mask, in which case, the localised heating step must disrupt both the dopant passivation layer and the dielectric layer. In this case, one of the layers may also act as an antireflection layer or a separate antireflection layer may be provided.
The first heating step is performed with parameters chosen to result in a surface region doping level in the range of 50-800 ohms per square and preferably in the range of 80-200 ohms per square.
The second localised heating step is preferably performed with a laser to enable concentrated heating in a small well defined area of the device. The laser may be a continuous wave laser or a pulsed (Q-switched) laser, however, in the latter case, the laser will generally be defocussed to prevent (or at least minimise) ablation during the heating step or else a wavelength at which laser energy will be absorbed close to the surface of the substrate.